# Shapekey driver miscalculates during playback and render?

video example, slightly NSFW

1. I am using corrective shapekeys for my ankles (see video link above).
2. The driver expression is the angle between two bone vectors.
3. Blender does notify me that it's "Slow python expression"
4. For example: poseBones[103].matrix.col[1] * poseBones[12].matrix.col[1]

I have many other drivers likes this in the scene. They all randomly bug out/don't update/delay. Both in render and viewport playback/edit. It's completely random.

1. Is it because of my slow expression?
2. Are there better workarounds?
3. Are there "non-slow python expressions" that can output the angle between two bones?

EDIT: My computer spec: 2060rtx 5900x 16gb ram SSD Tried on other computers with the same file. Same problem occurs, and again, the drivers bug out randomly.

• Hard to say on info given. blender.stackexchange.com/questions/134270/… There is a rotation difference driver variable type Which takes two target objects (bones) and returns the angle between in radians. Aug 30 '21 at 16:08
• which one is it? Can it give you the absolute angle and not just difference between selected axi? Aug 30 '21 at 16:18
• it gives the angle between. two bones like vector.angle(vector2) eg there is 90 degrees between z axis and both y and z. Could you elaborate on how you are using the resulting vector objtained by multiplying the pose bone matrix columns as outlined above? Aug 30 '21 at 17:03
• Aug 30 '21 at 21:17